Molarity of $4\%\left(\mathrm{w}/\mathrm{v}\right) \mathrm{NaOH}$ solution is

4% NaOH indicates that 4 g of NaOH present in 100 mL of solution. Moles of 4 g of NaOH:

$\mathrm{Moles}=\frac{\mathrm{Mass}}{\mathrm{Molar} \mathrm{mass}}=\frac{4 \mathrm{g}}{40 \mathrm{g}/\mathrm{mol}}$

Molarity is calculated as:

$$\begin{gathered} \mathrm{Molarity}=\frac{\mathrm{number} \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{solute}}{\mathrm{Volume} \mathrm{of} \mathrm{solution} (\mathrm{in} \mathrm{L})} \\ \mathrm{Molarity}=\frac{4 \mathrm{g}}{40 \mathrm{g}/\mathrm{mol}}\times \frac{1000}{100 \mathrm{mL}}=1.0 \mathrm{M} \end{gathered}$$